This invention relates to a steel cord for reinforcement of rubber products of a novel twisting structure to be used for reinforcement of rubber products such as automobile tires, conveyor belts, etc.
Generally, a steel cord used for reinforcement of rubber products such as tires, etc. is composed by intertwisting 3, 4 or 5 pieces of material wires into a strand (1.times.3, 1.times.4 or 1.times.5 construction) as shown in FIG. 7a to FIG. 7c. A steel cord (12) of this construction is composed by intertwisting a plural number of material wires (11) in close contact with one another and a cavity X is formed at the center of the cord. Such construction prevents good penetration of rubber compound into the cavity X at the central part of the steel cord at the time of vulcanization, leaving a continuous cavity as it is in the longitudinal direction of the cord. Tires using this type of steel cord, if they are damaged during the running of a car, produce a so-called "separation phenomenon" in which water penetrating through the damaged part reaches the cavity X and spreads in the longitudinal direction of the steel cord, thus corroding the steel cord and causing poor adhesion of steel cord to rubber compound to sharply reduce the service life of the tires.
However the development of wires of high strength in recent years has made it possible to reduce the number of material wires constituting the steel cord and, as a result, a steel cord for reinforcement of rubber of 1.times.2 construction as indicated in the official gazette No. PDS-62-234921 and No. PDS-62-117893 has also been developed (FIG. 8). In a steel cord (22) of this construction, the corrosion resistance of steel cord has been improved because the cavity X of the said steel cord (12) does not exist and the water penetrating through the damaged part of the tire does not spread in the longitudinal direction of the steel cord. However, a further improvement of fatigue resistance and flexibility is requested of steel cords to cope with the recent trend for higher speed of automobiles and meet the request for improvement of driving comfort. However, the said steel cord (22) of 1.times.2 construction was still poor in fatigue resistance and flexibility, because the material wires (21) which are in contact with each other are liable to produce fretting wear and no rubber compound penetrates into the portion of wire contacts forming a wire construction which seems as if it were made by twisting flat wires, leading to easy occurrence of buckling against repetitive compression and tensile and bending fatigue. Moreover the said steel cord was also unsatisfactory in the matter of corrosion resistance because the material wires which are in contact with each other cannot be covered completely with the rubber.